Regeneration and Changes in Synaptic Connections between Individual Nerve Cells in the Central Nervous System of the Leech

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Abstract
The central nervous system of the leech has been used for the study of the formation of new synaptic connections by regenerating neurons. In control leeches, individual nerve cells in adjacent ganglia are connected in an orderly and stereotyped manner, with only little variation. In the present experiments, a bundle of axons running between two of the segmental ganglia has been severed and allowed to regenerate. Subsequently, the axons reestablish synaptic connections between certain identified nerve cells in the adjacent ganglia, selectively and accurately. Thus, individual sensory cells in one ganglion show a high degree of neural specificity in reestablishing cell to cell connections with a motor cell in the next ganglion. The performance of the regenerated synapses, however, is significantly altered in a consistent manner. The normal balance between the effects of inhibitory and excitatory innervation in leeches with regenerated synapses is different from that seen in normal leeches, with marked overemphasis on inhibition. Similar alterations have also been seen in a series of ganglia at a distance from the site of the lesion. After the operation, therefore, a widespread modification of synapses occurs along the length of the nerve cord.